Single field polarizing spectrophotometer for measuring mass motion in a plasma



Apmi 25, 1967 J. G. HIRSCHBERG 3,316,412-

SINGLE FIELD POLARIZING SPECTROPHOTOMETER FOR MEASURING MASS MOTION IN APLASMA Filed June 9, 1964 2 Sheets-Sheet 1 {R T I F/ G [ll/l NW Q/j/////77/,

' EXIT SLIT PHOTOMULTIPLIERS ROCHON PRISM 23 25 ENTRANCE K SLIT 2|DIP-FRACTION GRATING PHOTO CATHODES/ I9 WOLLASTON PRISM H EXIT COMPOSITEsuT PLATE SPHERICAL MIRROR POLARIZATION DUOCHROMATOR INVENTOR. F/6.3JOSEPH G. HIRSCHBERG WMQ MJ A ni 25, 1%? G HIRSCHBERG 31,432

SINGLE FIELD POLARIZINC- SPECTROPHOTOMETER FOR MEASURING MASS MOTION INA PLASMA Filed June 9, 1964 2 Sheets-Sheet 2- FUSED CALCITE SHJCA EXIT 1DIRECTtON SLIT H OF CALCITE K OPTIC AXIS I I 29 rqykwi :}=IA

l v w v 1 @j COMPOSITE PLATES B+A (i.e.l3+l5) CHANNEU'A" CHANNEL "5"ETUDE STELLARATOR T w F/G.6

H0 cm/sec INVENTOR. JOSEPH G. H I RSCHBERG U ted at P wn SINGLE FEEL!)POLARHZKNG SPECTROPHOTOM- ETER FOR MEASURING MASS MOTION IN A PLASMAJoseph G. Hirschberg, Princeton, N.J., assignor to the United States ofAmericaas represented by the United States Atomic Energy CommissionFiled June 9, 1964, Seiu'No. 373,895

1 Claim. (Cl. 250-226) v ABSTRACTOF THE DISCLOSURE Apparatus formeasuring mass motion in a plasma by measuring the wavelength shift inan unpolarized beam .of light directed across the plasma in a narrowrange of wavelengths, wherein a monochromator transmits the light -beam,a first refracting prism splits the beam into equal perpendicularlypolarized rays, a retracting prism separates'the polarized rays, andphotosensitive means produces electrical signals corresponding to thewavelength .shift in the beam in passing through the plasma.

This invention relates to mass motion measurement and more particularlyto a method and apparatus for measuring mass motion in a hot gas orplasma.

. In plasma physics it is often desirable to have a simple device tomeasure the mass motion of a plasma such as the stellarator plasma,which emits a line spectrum. If w is the component of velocity of theemitter in the line of sight, from relativity we have the familiarformula for the frequency of the light received:

Only the first term w /c is ordinaraily significant, and:

[.t C A Thus to obtain w a shift in wavelength, often very small, is tobe measured.

The measurement of such a wavelength shift can in general only beperformed if the spectrum contains sharp lines. Let us consider such aline measured by a perfect monochromate with shape given by S0). Acentral wavelength, A exists such that The problem, therefore, is tomeasure small shifts AA from t Various proposals have been made and usedto accomplish such measurements including the use of spectrometershaving light pipes, rotating quartz plates and rapidly sweeping Kerreffect prisms. While these systems have been useful and can accomplishthe desired determinations, they do require the manufacture and assemblyof costly or undependable apparatus. Moreover, it has been desirable toprovide a system that does not require delicate alignment or complicatedelectronic gear and provides measurement of a band of wavelengths downto;

- C is the velocity of light.

Y. Patented Apr. .25, 1967 at the full resolving power of a grating;

It is still another objectof this invention to operate over a wide bandof wave lengths down to an ultraviolet wavelength of 2000 A. M

In accordance with this invention, there is provided a method andapparatus for measuring the plastic mass motion. in the stellarators atPrincetonUniversity-which move high and low density plasmas up to.maximum velocities and temperatures. The method and constructioninvolved in this invention utilizes standard and well known techniquesand apparatus and is highly flexible for a wide range of applications,energies, types of particles, and particle velocities, temperatures anddensities. More particularly, this invention, contemplates refractionmeans for measuring the wave length shift in a pass band beam .of lighthaving a narrow range of wave lengths, comprising a system of prisms forseparating the pass band beam into two polarized beams having a wavelength separation for conducting them into separate photosensitive meansfor producing electrical outputs corresponding to the wave length shiftin the pass-band. With the proper selection of refraction means andseparation as described in more detail hereinafter, the desiredmeasurements are provided.

The above and further objects and novel features of this invention willappear more fully from the following description when the same is readin connection with the accompanying drawings. It is to be expresslyunderstood, however, that the drawings are not intended as a definitionof the invention but are for the purpose of illustration only. I

In the figures, where like parts are marked alike:

FIG. 1 is a schematic diagram of a beam polarizer;

FIG. 2 is a partial cross-section of the beam separator for thepolarizer of FIG. 1;

FIG. 3 is a partial three-dimensional view of apparatus of FIG. 1 in thesystem of this invention;

FIG. 4 is a partial cross-sectional schematic of the composite plate ofFIG. 3;

FIG. 5 is a graphic illustration of wave length separation of the systemof FIG. 3;

FIG. 6 is a graphic illustration of a measurement of'a stellaratorplasma in accordance with this invention.

The particles present in the gas or plasma in the Etude stellarator atPrinceton University often have high velocities, which must bedetermined. Since the plasma energy, temperature and density aresufficient to produce a line spectrum, and these particles are movingwith different velocities, the Doppler effect somewhat broadens thisline spectrum over a range of wave lengths which corresponds with thevelocity of the particles.

The wave length shift AA in the spectral lines emitted is given by theexpression A \=u/C where A is the wave length, 11 is the velocity of theparticles (or a particle) and The shifts are generally of the order ofless than A.

In order to determine this shift in wave length as a function of timeand plasma position, the beam of light from the stellarator isintroduced into an Ebert monochromator where the pass-band beam of anarrow range p -by interposing crystals 11 of of wave lengths isproduced.- A doubly retracting crystal plate with optic axes at 45 tothe surface and in the horizontal is interposed between the entrance andexit slits in the monochromator. This has the effect of separating thepass band into two polarized halves, or rays having awavelengthseparation and also simultaneously conducts these through the exit slitof the monochromator. 'Advantageously, this plate is made from calcite,which is useful down to 2000 A.

In illustrating this above mentioned separation, reference is made toFIG. 1 whichshows two mutually perpendicularly polarized rays in plate11 comprising an optic ray 13 that'passes straight through the platellundeviated, and an extra-ordinary ray 15 that is deviated by an angle 6with the crystal or plate 11. 'Thetotal displacement, x of the two raysis .givenby -x=t sin. The. law of double refraction for a uniaxialcrystal is that. 7

tan 1 o mar he where r and r' arethe angles of the two polarized beamsas measured from the direction of the axis of the opt c-ray.

In the case shown r =45 and, therefore, r is equal to e/ao. For calcitewe=1.492 and no=1.670 at 5000 3.2. Thus, the trickness, V A. Separationwith a' A. The deviation t is about t of plate 11 required for a onemeter monochromator having a 30,000 groove per inch optical grating asdescribed in more detail hereinafter (x=25 microns), is nearly /2millimeter. With "acne-half meter monochromator and a 30,000 groove,Should these two rays 13 and 15 be conducted through a prism 17, whichto suitable standard photo-cathodes or means '19 and 21-as shown in FIG.2, the outputs from separates the rays and conducts them photomultiplerthe photocathodeswill correspond linearly with the amount of lightreceived, thewavelength shift A). from'v 1a and the mass plasma: motionor velocity. Except for adsorption and refraction losses, all the lightenergizing from the stellarator is thus utilized in one channelchromator between the entrance and exit slits. Advantageously, there aresix composite plates 11 with calcite of thickness such that a). is 0,0.1, 0.2, 0.4, 0.7, and 1.0 A. for 500 A. in first order. a

The composite plates 11 consist of a rectangular prism 33 of calcitejoined to a prism 35 of fused silica inserted so that the opticalthickness of the combination is the same for each plate 11. The opticalaxis of the calcite is orientated, also as shown in FIG. 4, so that theunpolarized pass-band light beam 29 enters the plate 11 and splits intotwo perpendicular plane-polarized rays or beams 13 and 15, which emergefrom plate 11 parallel to the entering beam 29; The extraordinary ray 15has a deviation angle 0 and the transverse displacement, 6x,

or the other." Moreover, there is no limitation tot-he 1 time resolutionexcept the response of the photomultican be determined at will i pliers,the channel operation different t, and the advantages of themonochromator have not been compromised in any way.

Apractical arrangement for accomplishing the wavelength shiftmeasurementin accordance with this invention is illustrated in FIG. 3.- The linespectrum or beam 23 of unpolarized light fromthe stellarator enterscollimator slit 25 of-monochromator 26, strikes spherical' mirror 27 andis reflected against grating 28, which di- 1 rects a narrow wavelengthportion of beam 23 against ,mirror 27 for the transmission of apass-band beam 29 having a narrow range of wavelengths throughcomposite.

plate 11 for wavelength separation. The entranceand exit slits 25 and 310f the monochromator 2 6 are adjustable,-providing flexibility forobtaining the bestconditions in observing narrow and broad lines. Thetwo channels are geometrically identical, having the same image andaperture-stops, thus avoiding the possibility,

of spatial changes in source light intensity being misinterpreted aswavelength shifts.

, Monochromator 26 is a commercial /2 meter Ebert- Fastie monochromator,such as the model 8200 made by the Jarrel Ash Co. Just behind the exitslit 31 is a slide containing quartz and calcite plates 11, 11 and 11"as shown in FIG. 4, arranged so that any one of 7 a number of plates 11can be interposed in the monoters entrance slit 25 from a stellarator(not shown).

Babinet-Soliel compensator before the entrance slit of the monochromator'26. Thus the action of the Zeeman eifect is eliminated.

After passing through the exit slit 31, a field lens (not shown) focusesthe polarized rays 13 and 15 on a prism 17 which makes aseparation intotwo diverging beams 41 and 43 which are focused by a suitable lens (notshown) on the separate photo-cathodes 19'and 21. This prism 17advantageously is a suitable Wollaston prism, but a'Rochon prism orother device to separate the two polarized beams may besubstitutedtherefor.

In operationa spectral line 23 of Gaussian (or near Gaussian) shape andwith a half-intensity breadth S en- The instrumental function of themonochromator 26 is substantially Gaussian with a half-intensity breadtht. The line measured is thus a Gaussian function of wavelength with ahalf-intensity breadth r given by r=.(S +t /2.

As shown inFIG; 5 the two neighboring channels A and B, described aboveas rays or beams are supplied, in effect by a duochromator B+A withseparation 61. The separation between the two channels a). has beenchosen so that the two functions A and B cross at the point of theirmaximum derivative. The wavelength change is proportional to thedifference signal, 1'=BA. Its derivative is the algebraic sum of thederivatives of A-and B, and thus is a maximumfor the 6A is given by:

For this example with Gaussion shapes,

may be used, the result being merely to divide the above result for dIby 2/ or about 1.2.

Where the functions involved are not exactly Guassians, the slope of Ior I' may be found experimentally by using a constant wavelength A andvarying A and A together. This is done simply by changing the setting ofthe monochromator by a known amount of d)\, and measuring the resultingdI or d".

Then:

d) A)\- -AI as before.

From an actual example of the described duochromator, or single fieldpola-chromator of this invention, FIG. 6 illustrates the strong 3 'P 'Dtransistion of singly ionized nitrogen at 3994.996 A., which was used tomeasure plasma rotation velocity in the Etude stellarator. Hydrogen gasat about 1 micron pressure was used with nitrogen added to provide astrong spectral line. The 3.8 cm. diameter limiter which touched theoutside diameter of the plasma was biased at +70 v. with respect to thedischarge chamber wall having a diameter of 7.6 cm. Velocities of up to5X10 cm./sec. are shown as a function of radius. The skew zero lineresulted from a zero correction evaluated with 6.\=0.

This invention has the advantage of measuring mass motion in a plasmawith simple, inexpensive and dependable refraction means and withoutdelicate alignment or complicated electronic gear. Moreover, it providesa spectrographic duochromator or single field pola-chromator formeasuring the small Doppler wavelength shifts in hot gasses and plasmasfor a wide range of applications, energies, types of particles andpanticle velocities, temperatures and densities contemplated instellarators.

What is claimed is:

Apparatus for measuring the wavelength shift in an unpolarized beam oflight having a narrow range of wavelengths, comprising monochromatormeans having a collimator, a grating and an exit slit for supplying andtransmitting a narrow wavelength portion of said beam through said exitslit, a first retracting prism in front of said exit slit for splittingsaid portion into equal perpendicularly polarized rays and transmittingsaid polarized rays simultaneously through said exit slit, a secondretracting prism for separating said polarized rays and transmittingthem, and separate photosensitive means for receiving said separatedpolarized rays for producing an electrical signal corresponding to theamplitude of said separated polarized rays after determining thewavelength shift in said unpolarized beam independently of intensityfluctuations in said unpolarized beam.

References Cited by the Examiner UNITED STATES PATENTS 2,471,249 5/1949Stearns et a1 8814 2,998,746 9/1961 Gievers 88-14 3,004,465 10/1961White 250226 X RALPH G. NILSON, Primary Examiner. J. D. WALL, AssistantExaminer.

